I have to go with J and teacake on this one. The only people who say money can't buy happiness are those who've never really been poor.
I've heard poor people say that out of bitterness, and rich people say it because they were spoiled and didn't appreciate what they had.
So the saying is pretty much empty and has no meaning.
Also the variations:
"Who cares about being rich, be glad you're healthy instead." Yeah well, get sick and recognize how much it helps being rich.